Summary: Respiratory syncytial virus is the most common cause of lower respiratory tract disease in young children worldwide. The F and G glycoproteins of this virus are known to be the major targets of a protective immune response, however, following natural infection, immunity is incomplete and repeated infections with viruses of homologous or heterologous subtypes occur despite high levels of neutralizing antibody. In addition, antibodies that neutralize extracellular virus may not be able to inhibit fusion of infected cells, allowing cell-to-cell spread. These findings suggest that B cell responses seen following infection may be deficient in antibodies that block specific virus-receptor interactions. In order to better understand virus-cell interactions required for infection, studies were initiated to more completely define the receptor(s) used by RSV. The binding of viral proteins to cell surface glycosaminoclycans was evaluated using heparin agarose affinity chromatography. Studies using whole virions,infected cell lysates, purified envelope glycoproteins, and lysates of vaccinia recombinant viruses expressing individual RSV F or G glycoprotein, showed that both F and G bind heparin. Furthermore, infected cells but not virions expressed and oligomeric complex of membrane associated glycoproteins F-G-SH that bound with higher affinity than either F or G alone. Over the past year we identified several linear F sequences responsible for heparin binding using overlapping, synthetic F peptides. These peptides were evaluated for their ability to bind in heparin agarose affinity chromatography, their ability to bind to Vero, Hep-2 and A549 cells, ability to block virus attachment and capacity to inhibit infection of cells with subgroup A and B strains. Binding to cells was specifically inhibited by soluble heparin and bovine lung heparin. During the past year we have also compared binding of peptides to untreated cells or to cells treated with GAG lysases using FACS analysis. Binding of F and G heparin-binding domain peptides was abrogated following enzyme treatment as indicated by both a decrease in the percentage of cells binding each peptide and a decrease in the median flourescent intensity. Binding to heparin sulfate deficient mutant CHO cell lines was decreased relative to the parental cells and was also diminished following chlorate treatment. Competitive binding studies on Vero cells showed that purified virus blocked binding of F and G heparin binding domain peptides. In contrast, reciprocal competition was not seen and peptide in excess did not block binding of purified virions. Taken together, these studies suggest that linear F and G heparin binding domain peptides attach specifically to cell surface glycosaminoglycans and that binding facilitates infection. Competitive binding studies also suggested that RSV binds to cell surface molecules other than GAGS, that F-heparin binding is a post-attachment event and that avidity of virion binding to cellular GAGS may be greater than peptide binding alone.